There are a wide range of applications where it is necessary to obtain both translational and rotational offsets between two bodies, preferably without physical contact. The prior art has accomplished such relative position sensing by using a variety of optical systems. Many have used combinations of light emitting diodes and photoreceptors to determine a relative position between two objects by sensing the amount of light falling on the photoreceptor. Such systems are shown in U.S. Pat. No. 4,766,322 to Hashimoto, U.S. Pat. No. 4,623,253 to Okutani, and U.S. Pat. No. 4,435,837 to Abernathy.
Others have employed video cameras and light sources, however, such devices are generally bulky, expensive and relatively slow, due to the video scan rate and time taken to process the data, (for instance, see U.S. Pat. No. 4,744,664 to Offt et al. and U.S. Pat. No. 4,396,945 to DiMatteo et al.).
More recently, position sensing systems have begun to use light emitting diodes with both four-quadrant position-sensitive sensors and lateral effect photodiodes. A four-quadrant sensor is a device having photosensitive receptors at four-quadrants each of which is separated by a small non-sensing region. So long as a beam spot is either centered in the non-sensing region or is equally centered on the four-quadrants, equal voltages are sensed and the position of the spot can be detected. Such a detector provides position information only up to the point where the edge of the spot reaches the detector gap. Thereafter, the spot is known to be in a particular segment but it is not known exactly where.
The lateral-effect photodiode comprises a single, active planar element. The position of the centroid of a light spot is derived by sensing photon-generated electrons within the substrate of the device. This is achieved by measuring the currents through multiple ohmic contacts around the periphery or on the back layer of the device. Ideally, the position of the light spot with respect to a middle axis separating two opposing lateral contacts is given by the ratio of the difference to the sum of the currents flowing through these contacts.
Such devices are made for both single dimensional and two dimensional tracking. The two dimensional tracking device has contacts placed about the four sides of a generally rectangular or square planar sensing area. The use of such types of sensors is generally discussed in an article by Kostamovaara et al., entitled "Method for Industrial Robot Tracking and Navigation Based on Time-of-Flight Laser Range Finding and the Position Sensitive Detection Technique". SPIE, Vol. 1010, Industrial Inspection (1988) pp. 92-99.
In U.S. Pat. No. 4,662,752 to Tucker et al. and U.S. Pat. No. 4,866,362 to Parker et al. target tracking systems are described wherein light beams corresponding to individual orthogonal axes are reflected off a target and onto individual, lateral-effect position sensing diodes (PSD's). The resultant of the signals from the reflected beams is used to indicate the target's position. In U.S. Pat. No. 4,576,481 to Hansen, a similar position sensing system using PSD's is shown wherein the detection of two or more targets may be accomplished by using light emitting diodes of different wavelengths, with each target having a retro reflector which includes a band pass filter that allows reflection of only one of the LED wavelengths.
Additional prior art has shown that PSD's can be used for both two and three-dimensional tracking of objects. By two dimensional tracking is meant the measurement of the position and orientation of an object constrained to move in a plane; by three-dimensional tracking is meant the measurement of all degrees of freedom (three translational and three rotational) of a rigid body moving in space. In all instances known to the inventor, such systems have suggested the use of a single, two dimensional PSD for two dimensional tracking and a plurality of PSD's for additional axes of movement. Such prior art can be found in U.S. Pat. No. 4,874,998 to Hollis Jr. and in "Robot Position and Orientation Sensor", Brennemann, Jr. et al., IBM TDB Vol. 26, No. 9, Feb. 1984, pp. 4457-4462 and "Non-Contact Sensor for Two-Dimensional Translation and Rotation", Hollis et al., IBM TDB Vol. 30, No. 7, Dec. 1987, pp. 32, 34.
While PSD's do provide excellent position sensing signals they are expensive and can range in cost from $90 for an active area of one centimeter by one centimeter, up to $2700 (active area 2 cm.times.2 cm) and higher. Thus it is most desirable that the number of PSD's required for positional sensing be minimized. Furthermore, digital processing is required to accomplish the mathematics required to determine the position of an object and an analog-to-digital converter channel is required for each PSD axis. As a result, limiting the number of PSD's enables substantial cost savings in associated electronic equipment. Finally, where multiple PSD's are employed for positional sensing, the mechanical alignment thereof can be difficult and is subject to error if not accomplished carefully.
Accordingly, it is an object of this invention to provide a three-dimensional position sensor which employs only a single two-dimensional PSD.
It is a further object of this invention to provide a three-dimension position sensor which is less costly and less prone to mechanical alignment errors than those exhibited in the prior art.
It is still another object of this invention to provide a six degree of freedom position sensor with unlimited motion range about one axis of rotation.